Multiperiodic linear accelerating structure

ABSTRACT

Multiperiodic linear accelerating structures comprising a succession of cylindrical accelerating cavities C a  having a revolution axis X 1  X 2 , and being coupled to each other by cylindrical coupling cavities C c  having the axis X 1  X 2  as revolution axis, which coincides with the mean path of the beam of the particles to be accelerated, the radius R c  of the coupling cavities C c  being substantially equal to the radius R a  of the accelerating cavities C a . Accelerating and coupling cavities are constituted by a stack of elements easy to machine and braze together.

The present invention relates to multiperiodic linear accelerating structures comprising a succession of accelerating cavities which are coupled to each other by orifices or coupling cavities. These coupling cavities may be disposed on the periphery of the accelerating cavities, or for smaller overall size, between these accelerating cavities. These coupling cavities are more specifically the subject of the present invention.

According to the invention there is provided a multiperiodic linear accelerating structure for accelerating a beam of charged particles, along an an axis X₁ X₂, by means of the action of an electromagnetic energy injected within said structure, said structure comprising a succession of accelerating resonant cavities of cylindrical shape and having the axis X₁ X₂ as revolution axis and coupling means for coupling two consecutive accelerating cavities, said coupling means comprising at least coupling cavities of cylindrical shape and having an axis X₁ X₂ as axis of revolution, each coupling cavity being disposed between two accelerating cavities, the radius of said coupling cavities being substantially equal to the radius of the accelerating cavities and the width of the coupling cavities, measured in the direction parallel to the axis X₁ X₂, being greater in the axial region, where the electrical component of the electromagnetic field produced by said electromagnetic energy is preponderant, than in the peripheral region.

For a better understanding of the invention and to show how the same may be carried into effect, reference will be made to the drawings accompanying the ensuing description in which:

FIG. 1 is a diagrammatic view of an accelerating structure according to the invention;

FIGS. 2 and 3 are sectional views of two embodiments of a biperiodic accelerating structure according to the invention;

FIG. 4 is a view of a triperiodic accelerating structure according to the invention.

The accelerating structure according to the invention shown in FIG. 1 comprises a succession of cylindrical accelerating cavities C_(a) of axis X₁ X₂ and coupling cavities C_(c) for coupling two consecutive accelerating cavities C_(a). These coupling cavities C_(c) have a radius R_(c) which is substantially equal to the radius R_(a) of the accelerating cavities C_(a) and are provided, in their centre, with an opening O for the passage of the beam of charged particles and, outside the central zone, with orifices T for coupling the coupling cavities C_(c) with the accelerating cavities C_(a) associated therewith.

To obtain a linear accelerating structure having a good efficiency per unit length, the coupling cavities C_(c) must be as narrow as possible. But the narrower these coupling cavities C_(c) the greater the increase in the inductance due to the coupling holes and therefore the smaller must be their radius R_(c) to obtain the suitable resonant frequency. In the accelerating structure according to the invention, the radius R_(a) of the accelerating cavities C_(a) and R_(c) of the coupling cavities C_(c) are equal, and this enables there accelerating structures to be constructed easily and with high precision, the excessive value of the inductance due to the coupling holes of the coupling cavities C_(c) being compensated for by an increase in the width of the coupling cavities C_(c), in the axial region.

FIG. 2 shows an embodiment of a biperiodic accelerating structure according to the invention. This structure comprises a stack of elements E₁ of cylindrical shape and axis X₁ X₂, the elements E₁ having at one of their ends a circular wall P₁ perpendicular to the axis X₁ X₂. These walls P₁ are placed in facing relation to each other and have such shape that they define therebetween, after assembly, a coupling cavity C_(c) of cylindrical shape and axis X₁ X₂. The central part of these walls is thickened and provided with an axial opening O₁ allowing the passage of the beam of particles to be accelerated and the orifice T₁ located outside the axis and permitting the coupling of the coupling cavity C_(c) and two accelerating cavities C_(a) associated therewith. The orifices T₁ opening into two successive accelerating cavities C_(a) are preferably located at 180° to each other, as shown in FIG. 2.

The elements E₁ are assembled by means of brazed joints J_(a) and J_(c).

The increase in the width of the coupling cavities in their central region is uniform in the embodiment shown in FIG. 2. Another embodiment of a biperiodic accelerating structure according to the invention is shown in FIG. 3. It is constituted by a stack of cylindrical elements E₂ having at one of their ends a circular wall P₂ provided with a central opening O₂ and an orifice T₂ outside the axis. The width of the central region of the coupling cavities C_(c) increases in a non-uniform manner.

FIG. 4 shows an embodiment of a triperiodic accelerating structure which is constituted by a stack of groups of cylindrical elements E₃, E₄, E₅. The elements E₃ and E₄ are identical and comprise respectively, at one of their ends, the circular walls P₃ and P₄ which are placed in facing relation to each other. The shape of the walls P₃ and P₄ is such that they define therebetween, when the elements E₃ and E₄ are assembled, a cylindrical coupling cavity C_(c) which widens in the axial region. The walls P₃ and P₄ are respectively provided with central openings O₃ and O₄ and coupling holes T₃ and T₄. The cylindrical element E₅ has, in its middle, a circular wall P₅ perpendicular to the axis X₁ X₂ are provided with a central opening O₅ and a coupling hole T₅ located outside the axis X₁ X₂. The rather thin walls P.sub. 3, P₄ and P₅ are thickened in the central region as shown in FIG. 4.

Such accelerating structures constituted by a stack of elements easy to machine and braze together are simple to construct and precise. 

What I claim is:
 1. A multiperiodic linear accelerating structure for accelerating a beam of charged particles along an axis X₁ X₂ by means of the action of an electromagnetic energy injected within said structure, said structure, comprising a succession of accelerating resonant cavities of a cylindrical shape and having said axis X₁ X₂ as revolution axis, and coupling means for coupling two consecutive accelerating cavities, said coupling means comprising at least coupling cavities C_(c), of cylindrical shape having the axis X₁ X₂ as axis of revolution, each coupling cavity C_(c), being disposed between two accelerating cavities C_(a), the radius R_(c) of said coupling cavities being substantially equal to the radius R_(a) of the accelerating cavities and the width of the coupling cavities, measured in a direction parallel to the axis X₁ X₂, being greater in the axial region, where the electrical component of the electromagnetic field produced by said electromagnetic energy is preponderant, than in the peripheral region.
 2. An accelerating structure as claimed in claim 1, wherein said structure is biperiodic.
 3. An accelerating structure as claimed in claim 2, wherein said structure is constituted by a stack of cylindrical elements E₁, respectively comprising, at one of their ends, a circular wall P₁, perpendicular to said axis X₁ X₂, the walls P₁, of two consecutive elements E₁, being disposed in pairs in facing relation to each other and having such shape that they define therebetween, after assembly, a coupling cavity C_(c), of cylindrical shape and said walls P₁ being provided with a central opening O₁, for the passage of said beam, and orifices T₁ located outside tha axis X₁ X₂, for coupling cavity C_(c) with the two accelerating cavities C_(a) associated therewith.
 4. An accelerating structure as claimed in claim 3, wherein said coupling cavities C_(c) have, in the axial region, a width increases in a uniform manner in the direction from the periphery toward the axis X₁ X₂.
 5. An accelerating structure as claimed in claim 3, wherein said coupling cavities C_(c) have, in the axial region, a width increases in a non-uniform manner in the direction from the periphery toward the axis X₁ X₂.
 6. An accelerating structure as claimed in claim 1, wherein said structure is triperiodic.
 7. An accelerating structure as claimed in claim 6, wherein said structure is constituted by a stack of groups of elements E₃, E₄, E₅ of cylindrical shape, the elements E₃ and E₄ being identical and comprising respectively, at one of their ends, a circular wall P₃ and a circular wall P₄ which are perpendicular to the axis X₁ X₂, said identical wall P₃ and P₄ being disposed in facing relation to each other and having such shape that they define therebetween, after assembly, a coupling cavity C_(c) of cylindrical shape, the element E₅ comprising in its middle a circular wall P₅ perpendicular to the axis X₁ X₂, said walls P₃, P₄, P₅ being provided in their center, respectively with openings O₃, O₄, O₅ for the passage of said beam, said walls P₃ and P₄ being provided respectively, outside the axial region, with coupling holes T₃ and T₄ for coupling each coupling cavity C_(c) with the two accelerating cavities C_(a) associated therewith, and said wall P₅ being provided, outside the axial region, with a hole T₅ for directly coupling the two accelerating cavities C_(a) located on each side of said wall P₅.
 8. An accelerating structure as claimed in claim 6, wherein said coupling cavities C_(c) have, in the axial region, a width increases in a uniform manner in the direction from the periphery toward the axis X₁ X₂.
 9. An accelerating structure as claimed in claim 6, wherein said coupling cavities have, in the axial region, a width which increases in a non-uniform manner in the direction from the periphery toward the axis X₁ X₂. 